The invention relates to the field of semiconductors, and, more particularly, to methods for making single crystal wafers and the wafers produced thereby.
Gallium nitride (GaN) is a highly desirable material for making many types of electronic devices. GaN has a wide bandgap of about 3.4 eV and is a direct-transistion type of semiconductor, and is thus attractive for light-emitting devices. It also has a high breakdown voltage, good transport properties and the ability to form high quality heterostructures. Accordingly, GaN is also attractive for high power, high temperature applications, such as high power amplifiers.
The desirable starting material for a GaN-based device would preferably be a bulk crystal or wafer form of GaN, on which various doped layers could be epitaxially grown. Unfortunately, GaN in wafer form is not producible using conventional melt pulling crystal growth techniques, as are silicon wafers, for example. Accordingly, approaches have been pursued for producing single crystal GaN films on growth substrates which remain attached to the GaN film to further serve as support or which are later removed.
For example, U.S. Pat. No. 5,625,202 to Chai discloses forming GaN on various substrate materials to produce light emitting devices. These substrate materials are described as modified wurtzite structure oxide compounds and include Lithium Aluminum Oxide, Sodium Aluminum Oxide, Lithium Gallium Oxide, Sodium Gallium Oxide, Lithium Germanium Oxide, Sodium Germanium Oxide, Lithium Silicon Oxide, Silicon Oxide, Lithium Phosphor Oxide, Lithium Arsenic Oxide, Lithium Vanadium Oxide, Lithium Magnesium Germanium Oxide, Lithium Zinc Germanium Oxide, Lithium Cadmium Germanium Oxide, Lithium Magnesium Silicon Oxide, Lithium Zinc Silicon Oxide, Lithium Cadmium Silicon Oxide, Sodium Magnesium Germanium Oxide, Sodium Zinc Germanium Oxide, and Sodium Zinc Silicon Oxide. The GaN layer remains on the growth substrate.
U.S. Pat. No. 6,156,581 to Vaudo et al. discloses growing one of a gallium, aluminum, or indium (Ga, Al, In) nitride layer on a substrate for subsequent fabrication, by metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) of a microelectronic device thereon. Vapor-phase (Ga, Al, In) chloride is reacted with a vapor-phase nitrogenous compound in the presence of a substrate to form (Ga, Al, In) nitride. The thickness of the base layer may be on the order of 2 microns and greater, and the defect density may be on the order of 108 cmxe2x88x922 or lower. The patent provides a laundry list of proposed foreign substrates including sapphire, silicon, silicon carbide, diamond, lithium gallate, lithium aluminate, zinc oxide, spinel, magnesium oxide, ScAlMgO4, gallium arsenide, silicon-on-insulator, carbonized silicon-on-insulator, carbonized silicon-on-silicon, gallium nitride, etc., including conductive as well as insulating and semi-insulating substrates, twist-bonded substrates (i.e., where the substrate of crystalline material is bonded to another single crystal substrate material with a finite angular crystallographic misalignment), and compliant substrates of the type disclosed in U.S. Pat. No. 5,563,428 to Ek et al., etc. The patent further discloses that in some embodiments, the substrate can be removed leaving a free-standing wafer. Unfortunately, the patent provides specific growth information only for sapphire.
U.S. Pat. No. 6,139,628 to Yuri et al. discloses forming GaN on sapphire by first heating the substrate in gas atmosphere including gallium, forming a first gallium nitride on the substrate, and forming a second gallium nitride on the first gallium nitride at a higher temperature than the temperature when the first gallium nitride was formed. A product of the gas including mono-atomic metal gallium is bonded to the surface of the substrate homogeneously and in a high density by applying heat to the substrate in the gas atmosphere including gallium. The first gallium nitride is formed at the low temperature by using the bonded metal gallium so that gallium nitride can complete its initial growth flat and homogeneously without re-vaporization. Further, after forming the first gallium nitride, the second gallium nitride is formed at the higher temperature, whereby the crystallinity of not only the first gallium nitride but also the second gallium nitride are improved through the heat treatment with the high temperature. After the third step is completed, the substrate may be removed.
A number of other approaches have also formed pretreatment layers prior to GaN deposition. For example, U.S. Pat. No. 6,086,673 to Molnar discloses forming a zinc oxide pretreatment layer on sapphire and then subjected to a gaseous environment including HCl and/or NH3 containing gas, that is thermochemically reactive with the zinc oxide. An epitaxial layer of GaN can be grown on the pretreated substrate.
Along these lines, U.S. Pat. No. 6,218,280 B1 to Kryliouk et al. discloses forming a nitrided layer on a lithium gallate substrate, forming a first GaN layer on the nitrided layer by metalorganic chemical vapor deposition (MOCVD), growing a next GaN portion using halide vapor phase epitaxy, and growing a capping GaN layer again using MOCVD. The GaN layers may then be separated from the substrate. The patent lists a number of other proposed substrates in addition to the specifically disclosed lithium gallate. These other substrates include LiAlO2, MgAlScO4, Al2MgO4 and LiNdO2. Unfortunately, the use of MOCVD results in carbon being incorporated into the GaN wafer. This carbon may be undesirable for many applications where pure GaN is desired.
An article by Naniwae et al. entitled xe2x80x9cGrowth of Single Crystal GaN substrate Using Hydride Vapor Phase Epitaxyxe2x80x9d in Jnl of Crystal Growth, Vol. 99, 1990, pp. 381-384, discloses growth of GaN films on a sapphire substrate. A pretreatment of gallium and HCl without ammonia for 10-20 minutes at 1030xc2x0 C. is used to pretreat the sapphire surface prior to metalorganic vapor phase epitaxy (MOVPE) of the GaN film.
An article by Xu et al. entitled xe2x80x9cxcex3-LiAlO2 single crystal: a novel substrate for GaN epitaxyxe2x80x9d in the Journal of Crystal Growth, Vol. 193, 1998, pp. 127-132, discloses LiAlO2 as a substrate for GaN film growth. The substrates were pretreated with ammonia, and thereafter the GaN film was grown using metalorganic chemical vapor deposition. Another article by Xu et al. entitled xe2x80x9cMOCVD Growth of GaN on LiAlO2 Substratesxe2x80x9d in Phys. Stat. Sol. (a) Vol. 176 (1999), pp. 589-593 also discloses an LiAlO2 substrate, an ammonia pretreatment, and MOCVD to form the GaN layer. Unfortunately, the MOCVD process may not be sufficiently fast to produce thicker films. In addition, the precursor gas for deposition is trimethylgallium which results in carbon being undesirably incorporated into the GaN layer.
An article by Waltereit et al. entitled xe2x80x9cNitride semiconductors free of electrostatic fields for efficient white light-emitting diodesxe2x80x9d in Letters to Nature, Vol. 406, Aug. 24, 2000, pp. 865-868, discloses the epitaxial growth of a thin layer of M-plane GaN on xcex3-LiAlO2 using plasma-assisted molecular beam epitaxy. The exposed surface of the thin GaN layer may be bonded to another substrate and the LiAlO2 layer then selectively removed to form certain types of higher efficiency devices.
An article also by Waltereit et al. entitled xe2x80x9cGrowth of M-Plane GaN(1{overscore (i)}00): A Way to Evade Electrical Polarization in Nitridesxe2x80x9d in Phys. Stat. Sol. (a) Vol. 180 (2000) pp. 133-138, similarly discloses the formation of an M-plane GaN layer on LiAlO2 substrate. The thin GaN layer (1.5 xcexcm sample) is grown using molecular beam epitaxy at a relatively slow growth rate of 0.5 xcexcm/h. The article reports that M-plane GaN is free of electrical polarization, as compared to more convention C-plane GaN, and that this leads to improved electron-hole wavefunction overlap and therefore improved quantum efficiencies. The M-plane GaN quantum wells have a dramatic improvement in room-temperature quantum efficiency, and the authors surmise that if contributions from competing non-radiative recombination channels are equal for M-plane and C-plane wells, then M-plane GaN opens the way for highly efficient ultraviolet emission.
Despite continuing developments in the area of GaN film growth, what would still be desired is an efficient approach to produce free-standing, high quality, single crystal, GaN wafers for use in subsequent device fabrication.
In view of the foregoing background, it is therefore an object of the present invention to provide a method for making high-quality, free-standing, single crystal GaN wafers for use in electronic devices.
This and other objects, features and advantages in accordance with the present invention are provided by a method for making a free-standing, single crystal, gallium nitride (GaN) wafer comprising forming a single crystal GaN layer directly on a single crystal LiAlO2 substrate using a gallium halide reactant gas, and removing the single crystal LiAlO2 substrate from the single crystal GaN layer to make the free-standing, single crystal GaN wafer.
Forming the single crystal GaN layer may comprise depositing GaN by vapor phase epitaxy (VPE) using the gallium halide reactant gas and a nitrogen-containing reactant gas. For example, the gallium halide reactant gas may comprise gallium chloride, and the nitrogen-containing reactant gas may comprise ammonia.
Because gallium halide is used as a reactant gas rather than a metal organic reactant, such as trimethygallium (TMG), the growth of the GaN layer can be performed using VPE which provides commercially acceptable rapid growth rates. In addition, the GaN layer is also devoid of carbon throughout. Because the GaN layer produced is high quality single crystal, it may have a defect density of less than about 107 cmxe2x88x922. Its major surface opposite the LiAlO2 substrate is also relatively smooth, such as having a surface roughness of less than about 5 nm RMS. Accordingly, the upper surface does not need a smoothing capping layer, such as also typically formed using a metal organic, such as TMG. Considered in somewhat different terms, the method may be considered as forming a single crystal GaN layer devoid of carbon directly on the single crystal LiAlO2 substrate.
Another aspect of the invention relates to pretreating the single crystal LiAlO2 substrate prior to depositing GaN which may enhance the quality of the GaN single crystal layer. More particularly, the pretreating may use the gallium halide reactant gas without the nitrogen-containing reactant gas. The pretreating may be performed for a time sufficient to form a monolayer of gallium on the single crystal LiAlO2 substrate. The pretreating and depositing may also be performed in the same chamber. Of course, the LiAlO2 substrate may be cleaned prior to forming the GaN layer.
The method may be advantageously used to produce a (1{overscore (i)}00)-oriented GaN wafer. Such a wafer offers advantages in terms of efficiency and producing UV spectrum light emitting devices. The (1{overscore (i)}00)-oriented GaN layer may be grown by using (100)-oriented tetragonal (xcex3) LiAlO2 as the starting substrate material.
An advantage of the LiAlO2 substrate is that it may be considered a compliant substrate, unlike sapphire, for example, that tends to cause wafers to take a bowed shape. Moreover, the method may include forming the GaN layer at an elevated temperature, and with the LiAlO2 substrate and the GaN layer having relative thicknesses so that the LiAlO2 substrate develops cracks therein upon cooling from the elevated temperature. These cracks may be advantageous for a subsequent wet etching step to remove the LiAlO2 substrate. The wet etching may comprise wet etching using hydrochloric acid at a temperature above room temperature.
The method may include the use of an LiAlO2 substrate having a diameter of 50 mm or greater so that the single crystal GaN wafers have a corresponding relatively large diameter. The GaN layer may also be grown to have a thickness of greater than about 100 xcexcm.
Another aspect of the invention relates to a free-standing, single crystal GaN wafer having characteristics different than prior art GaN wafers. More particularly, the GaN wafer may comprise (1{overscore (i)}00)-oriented, single crystal GaN which is devoid of carbon throughout, and which has a defect density of less than about 107 cmxe2x88x922. In addition, a major surface may have a relatively smooth surface with a surface roughness of less than about 5 nm RMS. The free-standing GaN wafer may have a diameter of greater than about 50 mm, and a thickness of greater than about 100 microns.
Another aspect of the invention relates to a method for making an electronic device, such as a light-emitting device, for example. The method preferably includes providing a (1{overscore (i)}00)-oriented, single crystal gallium nitride (GaN) layer being devoid of carbon and having a defect density of less than about 107 cmxe2x88x922; forming at least one doped semiconductor layer adjacent the (1{overscore (i)}00)-oriented, single crystal GaN layer; and forming at least one contact to the at least one doped semiconductor layer. A major surfaces of the (1{overscore (i)}00)-oriented, single crystal GaN layer may have a surface roughness of less than about 5 nm RMS. The (1{overscore (i)}00)-oriented, single crystal GaN layer may also have a thickness of greater than about 100 microns.
Still another aspect of the invention relates to an electronic device, such as a light-emitting device, for example. The electronic device preferably includes a (1{overscore (i)}00)-oriented, single crystal gallium nitride (GaN) layer being devoid of carbon throughout and having a defect density of less than about 107 cmxe2x88x922; at least one doped semiconductor layer adjacent the (1{overscore (i)}00)-oriented, single crystal GaN layer; and at least one contact to the at least one doped semiconductor layer.